1. Field of the Invention
This invention relates to an optical coupling device for transmitting light emitted by a first optical fibre to each of a plurality of second optical fibres.
Such a coupling device is disclosed in an article entitled POLYMERIC OPTICAL MIXING ROD COUPLER by L. L. Blyler and G. J. Grimes, of A. T. and T. Bell Laboratories, published in the proceedings of the 39th Electronic Conference, Houston, Tex., 1989, Pages 490 to 493. This known coupling device comprises a rectilinear mixing rod consisting of a Teflon (registered Trade Mark) tube filled with a transparent casting resin. A bundle of optical fibres is received in one end of the tube and a single large fibre is received in the other end thereof. The device may be operated as a light splitter or as a light combiner.
A further known optical coupling device, which is described in detail below, with reference to FIGS. 1 to 5 of the accompanying drawings, comprises first and second tubular connectors receiving the first and second optical fibres, respectively, with a cladded, optical end of each fibre exposed at a forward end of the respective connector, and an optical mixing rod interposed between the forward ends of the connectors, the mixing rod having optical ends optically interfaced with the optical ends of the first and second optical fibres, respectively, so as to distribute light emitted by the first optical fibre to illuminate the optical ends of the second optical fibres. The mixing rod of this further known optical coupling device, is rectilinear.
Such optical coupling devices, which are known as star couplers, are used for example in automotive vehicles, in optical instruments, for industrial automation, or for monitoring sensors in a machine, where it is required that a single light signal be transmitted to a plurality of light sensitive devices.
It has been found however, as described in greater detail below, that where such a rectilinear mixing rod is used, not all of the second fibres, that is to say the receiving fibres, are illuminated to an equal extent when the first fibre is emissive, so that the illumination of some of the second fibres may be inadequate. This disadvantage occurs, because the optical power transmitted by the mixing rod decreases in a direction away from the longitudinal central axis of the mixing rod.